In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as improved image quality and better control of contrast.
In helical scanning, and as explained above, only one view of data is collected at each slice location. To reconstruct an image of a slice, the other view data for the slice is generated based on the data collected for other views. Helical reconstruction algorithms are described in C. Crawford and K. King, "Computed Tomography Scanning with Simultaneous Patient Translation", Med. Phys. 17(6), Nov/Dec 1990, and in U.S. patent application Ser. No. 08/362,247, Helical Interpolative Algorithm For Image Reconstruction in A CT System, filed Dec. 22, 1994 and assigned to the present assignee.
When reconstructing an image for a particular slice, as the distance between the slice location and the location where the actual projection data was collected increases, the amount of error in the generated data for that slice also increases. These errors in the projection data usually causes low frequency shading artifacts. In sagittal or coronal image reformatting, these errors also cause fine horizontal streaks or artifacts. As with streak artifacts in axial imaging, the streaks in the reformatted images are annoying.
Since the projection data errors generally propagate in the horizontal (backprojection) direction, the streak artifacts have no correlation from slice to slice. That is, the artifacts are of high frequency in the z direction. Such artifacts can be removed by filtering the data in the z direction, however, when the true structure has rapid changes in the z direction, such as tissue air interfaces or bone-tissue interfaces, simply filtering the data in the z direction will result in a loss of image resolution.
It is desirable, of course, to reduce reconstruction artifacts. It also is desirable to reduce such reconstruction artifacts without adversely affecting image resolution.